The present invention relates to a drive system for a motor vehicle. Commercially available electric vehicles are distinguished by the fact that they can cover only a small range with one battery charge. Therefore, systems are increasingly offered which have a range extender (REX). This is generally a classic reciprocating piston internal combustion engine which drives a generator. The generated electrical energy is available to the drive system either directly or indirectly by buffering in an electrochemical battery. The range extender serves here to generate electrical energy and not to drive the vehicle directly.
In addition, there are so-called plug-in hybrid vehicles (PHEV). These also have an electric motor and a reciprocating piston internal combustion engine. However, these two units are generally coupled by means of a complex gear mechanism and, as a result, are able to drive the vehicle directly.
Electric vehicles with REX are so-called serial hybrids. The internal combustion engine is not directly used for propulsion here but instead charges the energy store via a generator. In this context, energy conversion losses are incurred since the mechanical power of the internal combustion engine is firstly converted by the generator into electrical power, and the electrical power is subsequently converted again into mechanical power by the electric traction motor. If the generated electrical energy is buffered, the efficiency level of the energy store must also be taken into account here.
In the case of PHEV vehicles, the internal combustion engine can drive the vehicle directly, but this usually requires a very complex gear mechanism in order to adapt the characteristic of the internal combustion engine to the driving function. These gear mechanisms are usually heavy and require a large amount of installation space.
DE 100 12 221 A1 discloses a hybrid vehicle with an internal combustion engine for driving a first axle and two electric machines for driving the first axle and a further axle.
Taking this prior art as a basis, an object of the present invention is to provide an improved drive system for a motor vehicle.
This and other objects are achieved by a drive system for a motor vehicle, a method of operating the drive system, as well as a vehicle equipped with the drive system, in accordance with embodiments of the invention.
According to one aspect, a drive system according to the invention for a motor vehicle has a first wheel pair driven by an internal combustion engine and an electric motor, a second wheel pair driven by an electric motor, the internal combustion engine which is coupled to the first wheel pair, a generator which can be operated as the electric motor and is coupled to the internal combustion engine and to the first wheel pair, and the electric motor which is coupled to the second wheel pair and is configured to convert energy generated by the generator into mechanical energy. In addition, a planetary gear mechanism is provided via which the internal combustion engine is coupled to the generator and to the first wheel pair.
The electric motor and the generator are preferably embodied as an asynchronous motor (ASM) or as a permanently excited synchronous machine (PSM). As a result, both can be operated either as a generator for generating energy or for the purpose of generating torque.
The internal combustion engine is coupled to the generator and to the output via the planetary gear mechanism. This arrangement makes it possible to transmit mechanical power to the output in a variable fashion. In this context, the rotational speeds of the internal combustion engine and of the generator can be varied. This degree of freedom permits particularly flexible adaptation of the electrical power provided by the generator to the demand of the on-board electrical power system.
The innovation consists, in particular, also in the arrangement of the individual components. The electric motor which is preferably arranged on the rear axle, i.e. the second wheel pair, can be embodied similarly to the manner in a pure electric vehicle. As a result, cost-effective standard solutions can be used here. By preferably adding the drive of the first wheel pair, which is based on an internal combustion engine, for example on the front axle, the possibility is provided of both extending the range of the vehicle with a low consumption of fuel, and of making available additional drive power. Both the consumption and the driving power can be improved compared to a classic electric drive by making the drive act on the first wheel pair. If the drive on the first wheel pair is dispensed with, a fully functionally capable drive is still available, which provides advantages in terms of the modularity.
The drive of the first wheel pair is preferably configured in such a way that it is possible to drive without difficulty at constant speeds up to the maximum speed and also with relatively low accelerations. This drive is therefore suitable for electric vehicles which travel over relatively large distances. Extremely small internal combustion engine power levels and generator power levels (coverage of the basic drive load) are already sufficient for this. When there is a high driving dynamic, the preferably relatively strong electric motor at the second wheel pair is then used. The drive system also provides the advantage that an all-wheel vehicle can preferably be implemented on the basis of a purely rear-wheel-drive vehicle.
According to one advantageous refinement of the drive system according to the invention, the first wheel pair is assigned to a front drive axle, and the second wheel pair is assigned to a rear drive axle. This provides the advantage that the assemblies which are assigned to the internal combustion engine, even the generator, can be accommodated in the front of the vehicle in the engine compartment, while only the relatively compact electric motor is accommodated at the rear. As a result, space advantages can be implemented.
According to a further advantageous refinement of the drive system according to the invention, a battery for storing and outputting energy is provided connected to the generator and the electric motor. As a result, better uncoupling of the generation of energy by the generator and the provision of drive torque by the electric motor can be implemented. A temporarily relatively low generation of energy by the generator, for example owing to a switchover process, can be compensated so that energy can be continuously retrieved by the electric motor.
According to a further advantageous refinement of the drive system according to the invention, a first brake is provided for braking, in particular securing, a ring gear of the planetary gear mechanism. As a result, the internal combustion engine can be particularly easily started by the generator. The first brake is embodied, for example, as a dog clutch, which permits freedom from slip in the closed state and also low frictional losses in the opened state.
According to a further advantageous refinement of the drive system according to the invention, a second brake is provided for braking, in particular securing, a web element of the planetary gear mechanism. This permits the generator to be able to be used additionally in the case of electric driving for driving the vehicle. This additional element additionally permits a recuperative braking torque by way of the generator on the first wheel pair, preferably on the front axle.
According to a further advantageous refinement of the drive system according to the invention, an uncoupling device is provided which is connected between the internal combustion engine and the first wheel pair. In the purely electric mode of operation, it is possible to uncouple the drive of the first wheel pair by way of the uncoupling device. This has the advantage that frictional losses which arise as a result of the driving of the elements in the planetary gear mechanism, for example the generator, are eliminated. This increases the efficiency level and makes it possible to implement a range in the electric driving mode which is only slightly restricted compared to a pure electric vehicle. The uncoupling device can be embodied as a dog clutch. As a result, the drag losses when the clutch is open can be minimized. The uncoupling device is preferably assigned to the first wheel pair and intermediately connected between the first differential and the first wheel pair. The uncoupling device can preferably be provided as a wheel hub clutch on the first wheel pair, which is advantageous with respect to moved masses, because the differential and the corresponding shafts are deactivated and a particularly large number of components can be deactivated by way of the uncoupling device. In addition, the uncoupling device can preferably be intermediately connected between the internal combustion engine and a first differential provided assigned to the first wheel pair, which permits relatively favorable utilization of the installation space to be achieved.
According to a further advantageous refinement of the drive system according to the invention, the generator is additionally embodied as a starter for the internal combustion engine. As a result, it is possible to dispense with an additional starter for starting the internal combustion engine. This is advantageous in terms of saving installation space, reducing the weight of the vehicle and reducing the vehicle costs. For this purpose, uncoupling is preferably carried out by way of the wheel hub clutch, and the first brake is closed and the second brake opened.
According to a further advantageous refinement of the drive system according to the invention, the drive system can preferably be switched into at least the following modes of operation: driving only the second wheel pair by use of the electric motor, wherein the internal combustion engine is uncoupled from the first wheel pair by way of the uncoupling device; driving the first wheel pair by use of the internal combustion engine or driving the first wheel pair by use of the internal combustion engine and driving the second wheel pair by use of the electric motor; driving the generator by use of the internal combustion engine, wherein the first wheel pair is uncoupled from the internal combustion engine by way of the uncoupling device, and the second wheel pair is uncoupled from the electric motor; and driving both wheel pairs by way of the electric motor and the generator, wherein the second brake is closed.
The aspects of the invention described above and the associated advantageous refinements of the drive system according to the invention also apply correspondingly, and vice versa, to the aspects of a vehicle described below, the method according to the invention and the advantageous refinements thereof.
A vehicle according to the invention has a drive system as described above.
According to one aspect, a method for operating a drive system has at least the following modes of operation: driving only the second wheel pair by use of the electric motor, wherein the internal combustion engine is uncoupled from the first wheel pair by way of the uncoupling device; driving the first wheel pair by use of the internal combustion engine or driving the first wheel pair by use of the internal combustion engine and driving the second wheel pair by use of the electric motor; driving the generator by use of the internal combustion engine, wherein the first wheel pair is uncoupled from the internal combustion engine by way of the uncoupling device, and the second wheel pair is uncoupled from the electric motor; and driving the first wheel pair by use of the electric motor and the second wheel pair by use of the generator, wherein the second brake is closed.
According to one advantageous refinement, in the mode of operation of driving the first wheel pair by use of the internal combustion engine or driving the first wheel pair by use of the internal combustion engine and driving the second wheel pair by use of the electric motor, the outputting of energy by the generator is controlled by adjusting the rotational speed of the internal combustion engine. This permits particularly flexible adaptation of the electrical power which is provided by the generator to the demand of the on-board electrical power system.
Further possible implementations of the invention also include combinations, not explicitly specified, of features which are described above or below with respect to the exemplary embodiments. In this context, the person skilled in the art would also add individual aspects as improvements or additions to the respective basic form of the drive system.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.